Hydrothermal growth and structural studies of Si(1-x)Ge(x)O2 single crystals

The substitution of germanium in the α-quartz structure is a method investigated to improve the piezoelectric properties and the thermal stability of α-quartz. Growth of α-quartz type Si(1-x)Ge(x)O(2) single crystals was performed using a temperature gradient hydrothermal method under different experimental conditions (pressure, temperature, nature of the solvent, and the nutrient). To avoid the difference of dissolution kinetics between pure SiO(2) and pure GeO(2), single phases Si(1-x)Ge(x)O(2) solid solutions were prepared and used as nutrients. The influence of the nature (cristobalite-type, glass) and the composition of this nutrient were also studied. Single crystals were grown in aqueous NaOH (0.2-1 M) solutions and in pure water. A wide range of pressures (95-280 MPa) and temperatures (315-505 °C) was investigated. Structures of single crystals with x = 0.07, 0.1, and 0.13 were refined, and it was shown that the structural distortion (i.e., θ and δ) increases with the atomic fraction of Ge in an almost linear way. Thus, the piezoelectric properties of Si(1-x)Ge(x)O(2) solid solution should increase with x, and this material could be a good candidate for technological applications requiring a high piezoelectric coupling factor or high thermal stability.